As a conventional vibration power generator, an electrostatic induction vibration power generation apparatus is known in which an electric charge is applied to one electrode of a variable capacity and the electric charge is induced to another opposed electrode through electrostatic induction. The electric charge to be induced is varied by changing of the capacity. The electrostatic vibration power generation apparatus serves to generate power by taking out the change in the electric charge as electric energy (see Patent Document 1).
FIG. 24 is a schematic cross-sectional view showing a conventional electrostatic vibration power generator disclosed in Patent Document 1, specifically, a vibration power generator 10 using electret material.
The vibration power generator 10 includes a first substrate 11 with a plurality of conductive surface regions 13, and a second substrate 16 with a plurality of electret material regions 15. The first substrate 11 and the second substrate 16 are spaced apart from each other by a predetermined distance such that the conductive surface regions 13 are opposed to the electret material regions 15. The second substrate 16 is fixed, and the first substrate 11 is coupled to fixed structures 17 via springs 19. The spring 19 is connected to each of both sides of the first substrate 11, and to the fixed structure 17. Even when the first substrate 11 is displaced by an external force, the spring 19 applies a restoring force to the substrate toward its original position, causing the substrate to reciprocate in the lateral direction (horizontal direction in the figure) and to return to the original position.
The displacement of the first substrate 11 causes fluctuations in the overlapped area between the electret material region 15 and the opposed conductive surface region 13, which results in a change in the amount of charge induced by the conductive surface regions 13. The vibration power generator 10 generates power by taking out the change in the amount of charge as the electric energy. A resonance frequency of vibration of the first substrate 11 is selected according to a frequency of the vibration to be used for power generation.
In the vibration power generator 10, however, the resonance frequency is determined depending on the states of the first substrate 11 and the spring 19, which disadvantageously makes it difficult to make the resonance frequency lower. In order to decrease the resonance frequency, it is necessary to increase the weight of the first substrate 11 or to decrease a spring constant of the spring 19. The spring 19 is normally formed of silicon or the like, which makes it difficult to decrease the spring constant of the spring 19 due to restrictions on the elastic constant of material or the size of the spring. For this reason, the weight of the first substrate 11 needs to be increased.
When the weight of the first substrate 11 is increased to make the resonance frequency lower, however, the spring 19 receives a large force produced by the vibration of the first substrate 11 (and the spring 19 is distorted largely). As a result, the spring 19 cannot be used for a long time. This leads to problems of insufficient durability and reliability of the vibration power generator 10.
In order to solve these problems, an electrostatic induction vibration power generator is proposed which can generate power from the vibration at a low frequency using a resin spring having higher resistance against elastic distortion (see Non-Patent Document 1).
FIG. 25 shows a schematic perspective view of an electrostatic induction vibration power generator using a resin spring as disclosed in Non-Patent Document 1. Referring to FIG. 25, the vibration power generator 20 includes a first substrate 21 with an electrode 23 including an electret film, a spring 29 for connecting each of both sides of the first substrate 21 to a fixed structure 27, and a second substrate 26 with an opposite electrode 25 formed thereover. The spring 29 is comprised of a parylene resin having the adequate durability, such as resistance to fatigue, and has a small elastic coefficient. Thus, the spring 29 enables the first substrate 21 to vibrate at a relatively low frequency with a high amplitude.
The spring 29 has a high aspect ratio structure in which its lengths in the directions perpendicular to an operating direction of the spring (that is, the directions y and z in the figure) is longer than that in the operation direction (that is, in the direction x in the figure). With this structure, the spring has a small spring constant in the operating direction and is likely to be deformed in the direction. In contrast, the spring has a large spring constant in the direction perpendicular to the operation direction, and thus is less likely to be deformed in the perpendicular direction. Thus, the spring is less likely to be bent in the directions other than the operation direction, so that the first substrate 21 is forced to vibrate only in the operating direction.